Electronic devices, such as, cellular telephones, laptop computers, keyless and wireless entry devices, and other integrated circuit based products may be powered by, for example, a battery, a fuel cell, a solar cell, a generator and the like require a stable and accurate voltage reference for effective and predictable operation. It is important that an accurate and reliable way of initiating the start-up and shut-down of electronic circuits in these battery powered devices over all possible operating conditions be used. During start-up (power-up) a minimum voltage value must be reached before beginning operation thereof, and during operation whenever a voltage goes below a critical value the device must be stopped or inhibited from further operation. Typical applications that perform these functions are power-on reset (POR) and power-low reset (or brown-out reset (BOR)).
POR and BOR circuits typically use a precision voltage reference in combination with a voltage comparator circuit(s) for determining if the critical voltage value has been reached at which a device may properly operate. Typical voltage references used in integrated circuits have been buried Zener and bandgap references. The buried Zener is a very stable and accurate voltage reference, however, it typically operates at about 5 volts or more and draws several hundred microamperes for optimum operation. The newer battery powered electronic systems may run at a battery voltage of 2 volts or less, thus, the buried Zener technique is not suitable as a voltage reference which must run from such a low voltage and also have low power consumption. For such applications a "bandgap reference" may be utilized.
A bandgap voltage reference, typically 1.2 volts, may be generated with a semiconductor circuit. This reference may be hereinafter referred to as a bandgap voltage and may be compensated for variations over changes in temperature by combining a negative temperature coefficient voltage circuit with a positive temperature coefficient voltage circuit to produce a substantially zero temperature coefficient voltage circuit, i.e., the bandgap voltage value remains substantially the same over a wide temperature range. A precision bandgap voltage reference circuit is more fully described in U.S. Pat. No. 5,900,773 by David M. Susak, and is incorporated by reference herein for all purposes.
A bandgap reference in combination with a voltage comparator may be used to sense operating voltage levels for POR and BOR circuits. The bandgap reference and comparator may be combined into one circuit such as disclosed in U.S. Pat. No. 5,781,043 by Willam Slemmer, and entitled "DIRECT CURRENT SUM BANDGAP VOLTAGE COMPARATOR" (referred to hereinafter as "Slemmer"). The Slemmer patent discloses a direct current sum bandgap voltage comparator for detecting voltage changes in a power supply. Upon detecting a power supply voltage level below a certain value, the Slemmer comparator will cause a transfer switch to change the power source to a backup battery. The Slemmer bandgap voltage comparator uses four current sources summed together to produce a summing node voltage level, and generates a logic signal that indicates when the summing node voltage is greater than or equal to, or less than a predetermined value. The predetermined value corresponds to a desired power supply voltage switchover value.
The prior art voltage references and comparator circuits are too complex, draw too much current, require operating voltage levels higher than are available in the newer battery operated electronic systems, and suffer from temperature and voltage stability variations. Thus, a need exists to provide an improved voltage comparator having low operating current, a stable voltage reference over a wide range of operating temperatures, and simple and reliable implementation in an integrated circuit.